As to a gland packing which is equipped in a stuffing box and which seals such as a stem in the stuffing box and inner wall surfaces of the stuffing box, hitherto, the cross sectional shape thereof has in general been a rectangle (square or oblong), a parallelogram, or a trapezoid having a right angle, and it has been generally known that at least any two of the gland packings are mutually attached closely, piled, and then used.
However, conventional gland packings have various problems. For example, when at least two gland packings of which all the cross sectional shapes are rectangles above are piled and equipped to a stem, there are problems such that: there is much scatter of the extension and deformation of the gland packings in the vertical directions (hereinafter, referred to as “in the directions of the inner and outer diameters”) to the tightening face pressure of the packing holder; and the exceedingly tightened portions and the loosely tightened portions easily coexist. Therefore, the tightening face pressure of the packing holder is inevitably increased exceedingly. Therewith, there is a demerit of increasing the moving resistance and required torque of the stem, or there is a fault such that: the gland packing materials are thrust into an opening at the stem side, and then protruded; and therefore the lowering of the sealing performance (sealability) and the leakage of enclosed materials are caused.
Accordingly, an attempt to facilitate the extension and deformation of the gland packing and to lower the tightening face pressure of the packing holder is made by comparatively lowering the density of the gland packing as used (namely, softening the gland packing). However, because of the character such that the extension and deformation amount in the directions of the inner and outer diameters of the aforementioned low-density gland packing is smaller than the compression and deformation amount in the direction of tightening the packing holder, it was necessary to further increase the tightening face pressure of the packing holder finally so that the stem would be sealed sufficiently. Furthermore, because the strain ratio (strain amount) of the aforementioned low-density gland packing is large, the number of gland packings necessary for sealing the stem may exceed an acceptable limit in the stuffing box, and besides there is a demerit of not achieving the function as a shaft receiver that prevents a moving error or vibration of the stem due to its softness, and there is also a problem of the permeation and leakage of the material itself because of the low density.
On the other hand, if the density is made high, the shape stability of the gland packing is improved, and the gland packing can be made excellent in such as functions as the shaft receiver. However, the extension and deformation amount in the directions of the inner and outer diameters to the tightening face pressure of the packing holder is fundamentally small. Therefore, it is necessary to further increase the tightening face pressure of the packing holder so that the stem would be sealed sufficiently. Therewith, caused are problems such that the sealing apparatus is enlarged and the handling convenience is lost.
As to the sealing apparatus, in addition to those which have a structure in which main packings of a kind are used in the above way, sealing apparatuses for such as stems in which at least two kinds of gland packings comprised of different materials or having different properties are mutually attached and equipped in a stuffing box have been well known hitherto. However, the reason that such a structure is selected is that at least two requisite or deficient performances (e.g. sealability, heat resistance, pressure resistance, and frictional resistance) are satisfied at a stroke. This was impossible for conventional gland packings of a kind alone. Therefore, in view of easiness and simplification of the sealing apparatus, there are problems such that at least two kinds of gland packings inevitably cannot help being equipped as the main packings.
In addition, U.S. Pat. No. 4,328,974 discloses a sealing apparatus so as to have a stress concentration in directions of the inner and outer diameters of a packing, in which a low-density gland packing of which the cross sectional shape is a parallelogram is placed between high-density gland packings of which the cross sectional shapes are trapezoids, and the shapes of both are made not to be mutually attached, and besides they are piled and equipped on a stem in a stuffing box, and thereafter the packings are mutually attached by tightening a packing holder. However, in the sealing apparatus according to this prior invention, there are various problems such that: 1) it is necessary to combine at least two packings having different densities; 2) low-density packings having a large strain ratio account for majority; 3) there is a stress concentration, but the number thereof is small; 4) in other than the stress concentration, the tightening face pressure of the packing is not transmitted more than that of a rectangular packing; and 5) the function of the low-density packing portion as a shaft receiver is lowered.